1. Field of the Invention
The present invention relates to an apparatus for measuring a fluid flow rate and more particularly to a fluid flow rate measuring apparatus useful for measuring a flow rate of intake air into an engine.
2. Description of the Prior Art
In a fuel injection engine, it is necessary to accurately measure a flow rate of intake air into the engine in terms of a weight flow rate of intake air and to control the rate of fuel supply to the engine at an optimum value which matches the air flow rate. An air flow rate measuring apparatus for this purpose is known in the art in which an electric heater resistance wire and two temperature dependent resistance wires are positioned in a suitable relation in the stream of intake air flowing into the engine and the air flow rate is measured by using signals generated from these resistance wires. In this apparatus, the intake air is heated by the electric heater resistance wire and the resulting rise in the temperature of the intake air is detected by the two temperature dependent resistance wires. Since the relation between the amount of heat applied to the intake air and the resulting temperature rise of the intake air vary in dependence on the weight flow rate of the intake air, the air flow rate can be measured on the basis of this principle.
This known air flow rate measuring apparatus has an advantage of being small in size and simple in construction and still capable of measuring the air flow rate accurately. However, the use of this apparatus in practical applications presents the difficult problem of how to support the resistance wires in the intake air stream.
In other words, since backfires occur frequently in an engine, the resistance wires must be supported in such a manner that they are not broken by the shock caused by any backfire. Also, in order to improve the accuracy and response of the flow rate measurement, it is very important to ensure that the amount of heat generated by the electric heater resistance wire is applied completely to the intake air and that the temperature rise of the intake air is detected quickly by the two temperature dependent resistance wries.